Since their discovery in 1991, carbon nanotube is a fascinating subject for curiosity-driven research. This carbon material is a graphene cylinders called carbon nanotube (CNT). There are two main CNT varieties: Multi-Walled NanoTubes (MWNT), are collections of several concentric graphene cylinders and are larger structures compared to Single-Walled NanoTubes (SWNTs) which are individual cylinders.
These structures, produced by three main methods process: arc discharge (C. Journet and al. in Nature (london), 388 (1997) 756), laser furnace (A. G. Rinzler and al. in Appl. Phys. A, 1998, 67, 29) and chemical vapor deposition (P. Nicolaev and al. in Chem. Phys. Lett., 1999, 313, 91) make them a very unique material with a whole range of promising applications.
They have the right combination of properties—nanometersize diameter, structural integrity, high electrical conductivity, high mechanical properties and chemical stability. There have been some successes in several domains demonstrating potential applications of nanotubes.
This is well reflected in the literature as well as in the huge number of patents filled worldwide: electronic and electrochemical applications of nanotubes, nanotubes as mechanical reinforcements in high performance composites, nanotube-based field emitters, and their use as nanoprobes in metrology and biological and chemical investigations, and as templates for the creation of other nanostructures, electronic properties and device.
For background information and some application patents see WO 91/03057 U.S. Pat. No. 5,744,235, U.S. Pat. No. 5,445,327, U.S. Pat. No. 54,663,230, B. I. Yakobson and R. E. Smalley, American Scientist, 85 (July-August, 1997) pp. 324-337.
Carbon nanotubes have substantial potential for enhancing the strength, elasticity, toughness, electrical conductivity and thermal conductivity of polymer composites, however incorporation of the nanotubes into composites has been complicated by the tendency to aggregate and impair dispersion of the nanotubes.
Generally, preparation of most polymer-multi-wall-carbon composites has been directed to achieve a uniform carbon nanotube dispersion in polymers using such methods as mechanical mixing, melt-blending, solvent blending, in-situ polymerization and combinations thereof.
Generally homogenous aqueous dispersions of single-wall carbon nanotubes have been prepared by using certain water-soluble polymers that interact with the nanotubes to give the nanotubes solubility in aqueous systems. (See M. J. O'Connell et al., Chem. Phys. Lett. 342 (2001) p. 265). Such systems are described in International Patent Publication, WO 02/16257, published Feb. 28, 2002.
Conductive polymer composites containing carbon-based fillers are desired for their unique combination of metallic conductivity and polymer flexibility. Such conductive polymer composites are useful as materials for electromagnetic interference (EMI) shielding, heat dissipation films, paints, coatings, adhesives, chemical sensors, actuators, photoconductors, and impedance adapters for organic light emitting diodes (OLEDs).
One carbon-based filler that has been used in polymer composites is carbon black. However, in order to achieve the desired electrical conductivity with carbon black, concentrations of more than 10 wt % are often needed in the polymer when processed by typical solution or melt-based techniques.
High filler loadings, such as these, can result in processing difficulties and loss of polymer properties, such as flexibility. Generally this result obliges to reformulate the polymer to recover mechanical properties.
The critical filler concentration needed to achieve true electrical conductivity is known as the percolation threshold. A need remains for a conductive polymer composite with a percolation threshold at a low critical filler concentration in order to retain polymer properties (mechanical, optical, surface aspect . . . ) and processability, as well as provide a composite with effective conductive properties.
The percolation threshold is the critical concentration of conductive fillers needed to pass the polymer (or formulation) from insulator to conductive state. The main parameters to achieve low percolation threshold are: good dispersion and high carbon nanotube aspect ratio.
WO 2004/097853 provides a conductive carbon nanotube-polymer composite which comprises carbon nanotubes and polymer, wherein the polymer is in a form of coalesced polymer particles, wherein the carbon nanotubes reside primarily between the polymer particles, and wherein the carbon nanotubes form an interconnecting network at the interface between at least some of the coalesced polymer particles. The network of carbon nanotubes in the carbon-nanotube polymer composite provides electrical and thermal conductivity to the composite.
There is still a need for improving the electrical and thermal conductivity of such carbon nanotube-polymer composite.